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.TH PCAP_GET_SELECTABLE_FD 3PCAP "22 July 2011"
.SH NAME
pcap_get_selectable_fd \- get a file descriptor on which a select() can
be done for a live capture
.SH SYNOPSIS
.nf
.ft B
#include <pcap/pcap.h>
.ft
.LP
.ft B
int pcap_get_selectable_fd(pcap_t *p);
.ft
.fi
.SH DESCRIPTION
.B pcap_get_selectable_fd()
returns, on UNIX, a file descriptor number for a file descriptor on
which one can
do a
.BR select() ,
.BR poll() ,
or other such call
to wait for it to be possible to read packets without blocking, if such
a descriptor exists, or \-1, if no such descriptor exists.  Some network
devices opened with
.B pcap_create()
and
.BR pcap_activate() ,
or with
.BR pcap_open_live() ,
do not support
.B select()
or
.B poll()
(for example, regular network devices on FreeBSD 4.3 and 4.4, and Endace
DAG devices), so \-1 is returned for those devices.
.PP
Note that a descriptor on which a read can be done without blocking may,
on some platforms, not have any packets to read if the read timeout has
expired.  A call to
.B pcap_dispatch()
will return 0 in this case, but will not block.
.PP
Note that in:
.IP
FreeBSD prior to FreeBSD 4.6;
.IP
NetBSD prior to NetBSD 3.0;
.IP
OpenBSD prior to OpenBSD 2.4;
.IP
Mac OS X prior to Mac OS X 10.7;
.PP
.B select()
and
.B poll()
do not work correctly on BPF devices;
.B pcap_get_selectable_fd()
will return a file descriptor on most of those versions (the exceptions
being FreeBSD 4.3 and 4.4), but a simple
.B select()
or
.B poll()
will not indicate that the descriptor is readable until a full buffer's
worth of packets is received, even if the read timeout expires before
then.  To work around this, an application that uses
.B select()
or
.B poll()
to wait for packets to arrive must put the
.B pcap_t
in non-blocking mode, and must arrange that the
.B select()
or
.B poll()
have a timeout less than or equal to the read timeout,
and must try to read packets after that timeout expires, regardless of
whether
.B select()
or
.B poll()
indicated that the file descriptor for the
.B pcap_t
is ready to be read or not.  (That workaround will not work in FreeBSD
4.3 and later; however, in FreeBSD 4.6 and later,
.B select()
and
.B poll()
work correctly on BPF devices, so the workaround isn't necessary,
although it does no harm.)
.PP
Note also that
.B poll()
doesn't work on character special files, including BPF devices, in Mac
OS X 10.4 and 10.5, so, while
.B select()
can be used on the descriptor returned by
.BR pcap_get_selectable_fd() ,
.B poll()
cannot be used on it those versions of Mac OS X.  Kqueues also don't
work on that descriptor.
.BR poll() ,
but not kqueues, work on that descriptor in Mac OS X releases prior to
10.4;
.B poll()
and kqueues work on that descriptor in Mac OS X 10.6 and later.
.PP
.B pcap_get_selectable_fd()
is not available on Windows.
.SH RETURN VALUE
A selectable file descriptor is returned if one exists; otherwise, \-1
is returned.
.SH SEE ALSO
pcap(3PCAP), select(2), poll(2)
